During the past year, we have continued to study the structure of chromatin, the complex of proteins and DNA found in Eukaryotic nuclei. Our earlier work had suggested that the DNA of nucleosome might be quite accessible to the surrounding enviornment. We have confirmed this using the small chemical probe, dimethyl sulfate. Except for one exceptionally reactive region, the DNA of the nucleosome appears to be as accessible as DNA in solution. We have also been able to construct nucleosome-like particles using T4 phage DNA, in which the large groove is blocked by glucose residues. As a tool for the study of the structure of transcriptionally active regions of chromatin, we have cloned a 6 kilobase genomic fragment containing the chicken adult Beta globin gene, and have mapped the fragment using restriction enzymes, and electron microscopy. Using restriction enzymes, a site near the 5' end of the globin gene has been shown to undergo remarkable changes in extent of methylation (5 position of cytosine) in cells which have a history of globin gene expression.